A diachronic archaeometallurgical study of Chengdu Plains bronzes (Li et al. 2020, Journal of Cultural Heritage 43) established a two-axis material signature for local bronze production from the Western Zhou to the Warring States period — a copper signal dominated by Oxford Copper Group 4 (silver-bearing) together with lead isotope fields that change between periods — and showed that the earlier Sanxingdui sacrificial-pit bronzes match neither axis, carrying highly radiogenic lead and a different copper signal instead. The study left open the identity of the ore deposits behind these signatures, the production locus and metal procurement mode of the Sanxingdui bronzes (locally made with imported metal versus made elsewhere), the disentangling of typological, chronological, and supply-chain determinants of alloying, and the metal circulation of post-Warring States periods. Literature through early 2026 has expanded sampling, sharpened the debate on the source of highly radiogenic lead (Qinling, North China Craton margins, Yunnan, and multi-source models all proposed or contested), and produced new Sanxingdui analyses (casting-core petrography; 79 new samples from Pits 1–2, all highly radiogenic), yet explicitly leaves raw-material provenance unresolved: no ore source has been identified for either the local Chengdu Plains signatures or the Sanxingdui highly radiogenic lead, and no in-situ production evidence has been found at Sanxingdui. The problem remains open.

Background

The Chengdu Plains of Sichuan, enclosed between the Tibetan Plateau and the western Sichuan Basin, host one of China's most distinctive Bronze Age cultural sequences, centred on the walled site of Sanxingdui (famous for two sacrificial pits excavated in 1986 and six more excavated in 2019–2022, containing hundreds of monumental bronzes together with gold, jade and ivory) and its successor settlement Jinsha. Unlike most of Bronze Age China, this region lay outside the Shang political sphere, so the origin of its bronzes — locally developed, or connected to the Shang heartland by trade, migration, or imported metal — has been debated since the 1980s.

The provenance of ancient bronze is usually approached by lead isotope analysis: lead has four stable isotopes whose relative abundances in an ore depend on the deposit's geological age and uranium/thorium environment, so ratios such as 206Pb/204Pb^{206}\mathrm{Pb}/^{204}\mathrm{Pb} measured in a bronze can in principle be matched to the ore body that supplied its lead. Because bronze is an alloy of copper with tin and lead, and lead was normally added deliberately, the isotope signal tracks the lead source and, at low lead contents, partly the copper source. A complementary tool is trace-element "Copper Group" classification, an Oxford scheme that records which of four trace elements — arsenic (As), antimony (Sb), silver (Ag), nickel (Ni) — exceed a 0.1% threshold in the metal: for example CG4 marks silver alone and CG7 marks silver plus antimony. This characterizes the copper source independently of the deliberately added lead.

A long-standing enigma of this field is highly radiogenic lead (HRL): many Shang-period (c. 1500–1046 BCE) bronzes across China — from the Shang capital at Anyang to Sanxingdui — contain lead with unusually high proportions of radiogenic isotopes (206Pb/204Pb^{206}\mathrm{Pb}/^{204}\mathrm{Pb} well above about 19), pointing to very ancient, uranium-rich source rocks. Despite three decades of debate the deposit or deposits that supplied this lead have never been identified: proposals include southwestern China (Yunnan–Guizhou–Sichuan), the middle Yangtze valley, the Qinling mountains, the margins of the North China Craton, and even Africa, several of which have been argued to lack supporting evidence, and a multi-source model has also been advanced.

Within this context, Li et al. (2020) analysed 37 Chengdu Plains bronzes spanning the Western Zhou (1046–771 BCE) to the Tang dynasty (elemental compositions by ICP-AES, the inductively coupled plasma emission spectroscopy used for quantitative bulk composition; metallography, i.e. microscopic examination of microstructure; and lead isotopes), and compared them with published Sanxingdui pit data. They found that locally produced bronzes from the Western Zhou to the Warring States period (475–221 BCE) share one copper signal — Copper Groups dominated by CG4 and CG7 — while their lead isotope fields differ between the Jinsha (Western Zhou–Spring and Autumn) and the Warring States Baishoulu cemetery assemblages, implying different lead sources over more than six centuries; some Han and Tang coins may reuse the same lead sources. The Sanxingdui bronzes match neither axis: they carry highly radiogenic lead and copper of Copper Groups CG1 and CG2 (two further groups in the same Oxford scheme), so both their copper and their lead differ from the later local pattern. The authors concluded that the Sanxingdui bronzes "might not have been locally made or was made with outside materials". Subsequent methodological work has emphasized that mixing and recycling of metal in antiquity complicate such attributions, since the isotope signature of a bronze reflects its last-added lead rather than the whole metal inventory.

Problem Statement

Determine the metal sources and production organization underlying bronze production in the Chengdu Plains, arising from the findings of Li et al. (2020) and still open in the literature. Concretely: (i) locate and geochemically characterize the copper and lead ore deposits that supplied the locally produced post-Sanxingdui bronzes — the source of the dominant CG4 (silver-bearing) copper trace-element signal and of the lead sources that changed between the Jinsha (Western Zhou–Spring and Autumn) and Warring States periods; (ii) determine where the Sanxingdui sacrificial-pit bronzes were produced and from what metal — locally made using metal imported from outside the Chengdu Plains, cast elsewhere and imported as finished objects, or a combination — including the ultimate geological origin of their highly radiogenic lead; (iii) establish whether observed differences in alloying practice reflect object function and typology, chronological change, or different metal supply chains, rather than confounding among these factors; and (iv) extend the diachronic baseline beyond the Warring States period, including coins and Qin, Han and later bronzes, so that the end of the local production tradition and subsequent metal circulation can be characterized. The scientific target is an attribution of the Chengdu Plains bronze signatures warranted by a match to a specific ore deposit, wherever the bronze and ore data permit one, or by a named supply system together with the evidence that identifies it, a defensible determination of the production locus and metal procurement mode of the Sanxingdui pit assemblage that explicitly accommodates object-class and stylistic-group heterogeneity (different groups may have had different production loci), a causal attribution of differences in alloying practice to object function and typology, chronology, or different metal supply chains rather than confounding among these factors, and a coherent account of how the metal supply system evolved across the periods in question.

References

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